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We constrain the spatial gradient of star formation history within galaxiesusing the colour gradients in NUV-u and u-i for a local spatially-resolvedgalaxy sample. By splitting each galaxy into an inner and an outer part, wefind that most galaxies show negative gradients in these two colours. We firstrule out dust extinction gradient and metallicity gradient as the dominantsource for the colour gradient. Then using stellar population models, weexplore variations in star formation history to explain the colour gradients.As shown by our earlier work, a two-phase SFH consisting of an early secularevolution (growth) phase and a subsequent rapid evolution (quenching) phase isnecessary to explain the observed colour distributions among galaxies. Weexplore two different inside-out growth models and two different inside-outquenching models by varying parameters of the SFH between inner and outerregions of galaxies. Two of the models can explain the observed range of colourgradients in NUV-u and u-i colours. We further distinguish them using anadditional constraint provided by the u-i colour gradient distribution, underthe assumption of constant galaxy formation rate and a common SFH followed bymost galaxies. We find the best model is an inside-out growth model in whichthe inner region has a shorter e-folding time scale in the growth phase thanthe outer region. More spatially resolved ultraviolet (UV) observations areneeded to improve the significance of the result.

Inside-out growth or inside-out quenching? clues from colour gradients of local galaxies